Messages in this thread | | | From | Frederic Weisbecker <> | Subject | [RFC][PATCH 0/2] cgroup: Fix some races against css_set task links | Date | Wed, 8 Feb 2012 03:37:25 +0100 |
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Hi,
This is trying to fix some races in the way to link all the tasks to the css_set links. I hope some people can have a look at this, especially as I'm definetly not an SMP ordering expert.
To give you the big picture, as long as nobody never calls cgroup_iter_start(), we don't link the tasks to their css_set (this 'link' is a simple list_add()).
But once somebody calls cgroup_iter_start(), we call cgroup_enable_task_cg_lists() that grossly does this:
cgroup_enable_task_cg_lists() { use_task_set_css_links = 1; do_each_thread(g, p) { link p to css_set } while_each_thread(g, p); }
But this links only existing tasks, we also need to link all the tasks that will be created later, this is what does cgroup_post_fork():
cgroup_post_fork() { if (use_task_set_css_links) link p to css_set }
So we have some races here:
- cgroup_enable_task_cg_lists() iterates over the tasklist without protection. The comments are advertizing we are using RCU but we don't. And in fact RCU doesn't yet protect against while_each_thread().
- Moreover with RCU there is a risk that we iterate the tasklist but we don't immediately see all the last updates that happened. For example if a task forks and passes cgroup_post_fork() while use_task_set_css_links = 0 then another CPU calling cgroup_enable_task_cg_list() can miss the new child while walking the tasklist with RCU as it doesn't appear immediately.
- There is no ordering constraint on use_task_set_css_links read/write against the tasklist traversal and modification. cgroup_post_fork() may deal with a stale value.
The second patch of the series is a proposal to fix the three above points. Tell me what you think.
Thanks.
Frederic Weisbecker (2): cgroup: Remove wrong comment on cgroup_enable_task_cg_list() cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list
kernel/cgroup.c | 23 ++++++++++++++++++++--- 1 files changed, 20 insertions(+), 3 deletions(-)
-- 1.7.5.4
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